1. Field of the Invention
The present invention relates generally to a fabrication process for manufacturing semiconductor devices. More particularly, the present invention relates to a method and etchant useful for etching TiSix.
2. Background
Titanium silicide (TiSix) is a material likely to be used with increasing frequency in semiconductor devices. In particular, TiSix is a candidate for use as a gate material in memory devices such as DRAMs and SRAMs, because it favorably has a lower resistivity than materials presently used as gate materials in such devices, such as tungsten silicide (WSix) and polysilicon. This lower resistivity allows faster devices, i.e., devices having a smaller RC delay, to be fabricated. Moreover, it is becoming possible to fabricate memory devices having dimensions smaller than about 0.25 microns. At this small scale, the resistance of the conventional gate electrode composed of polysilicon and tungsten silicide (WSix) will be high enough to cause significant delay in signal transmission. Substituting TiSix for the WSix is expected to advantageously mitigate such delay.
Many desirable applications require the ability to anisotropically etch TiSix. It is known to anisotropically etch TiSix using Cl2 or HBr in an inductively coupled plasma (ICP) etcher. See Soda et al., xe2x80x9cCorrelation between Etching Characteristics of Titanium Silicide and its Crystal Structure under Cl2 or HBr Plasma Etching,xe2x80x9d Dry Process Symposium, 33, 1997, (xe2x80x9cSodaxe2x80x9d) which is hereby incorporated by reference in its entirety. However, it has been observed that such etching may leave a residue due to the presence of Si nodules in the TiSix. See Soda. This residue is very difficult to remove, and may interfere with device performance. In addition, the Si nodules may act as xe2x80x9cmicro-masksxe2x80x9d that shield underlying TiSix from the etching process, which can lead to roughness in the profile of the etched surface. Also, many process conditions may lead to undercutting, due to isotropic etching of the sidewall.
A method of etching TiSix is provided. A patterned mask is provided on top of a layer of TiSix. The TiSix is exposed, through the patterned mask, to a plasma etchant. The plasma etchant comprises (i) at least one fluorine containing gas, such as SF6, NF3, CxFy (where x ranges from about 1 to about 10), and compatible mixtures of such gases; and (ii) a gas selected from the group consisting of HBr, Cl2, and combinations thereof.
Conventional methods of etching TiSix use Cl2 or HBr as the plasma etchant. However, these methods can lead to undesirable residues, due to the presence of silicon nodules in the present TiSix. While not intending to be limited to any explanation as to why the present invention works, it is believed that the conventional Cl2 or HBr plasma chemistry has a selectivity for TiSix, relative to Si nodules, significantly greater than 1, ie., the plasma etches TiSix at a faster rate than Si nodules that are often present in TiSix. Since the Si nodules are not etched as quickly as the TiSix, they act as xe2x80x9cmicro-masks,xe2x80x9d by shielding underlying TiSix from the etching process. The Si nodules may also act as micro-masks for other layers, such as a layer of polysilicon underlying the TiSix. Note that the Si nodules are not polysilicon, and are etched at rates significantly different from polysilicon. The difference in the etch rate of the Si nodules and TiSix is more pronounced when Cl2 is used as the etchant than when HBr is used. As a result, the residue and micro-masking effect are more severe when Cl2 is used as the etchant. The present invention overcomes the residue problem by adding a fluorine containing gas to the plasma etchant, which is then able to effectively etch the Si nodules so that the undesirable residue is not formed.